1. Field of the Invention
The invention relates to a magneto-optical recording medium for recording and reproducing information by a laser beam by using a magnetooptic effect and, more particularly, to a magneto-optical recording medium for enabling a high density of a medium to be realized and to an information reproducing method for such a medium.
2. Related Background Art
As a rewritable high density recording system, attention is paid to a magneto-optical recording medium such that information is recorded by writing magnetic domains to a thin magnetic film by using thermal energy of a semiconductor laser. Information is read out by detecting a change in Kerr rotational angle of a reflected light from the medium. In recent years, a demand to raise a recording density of the magneto-optical recording medium and to realize a recording medium of a further large capacity has increased.
The linear recording density of an optical disk such as a magneto-optical recording medium or the like largely depends on a laser wavelength .lambda. of a reproducing optical system and a numerical aperture NA of an objective lens. Namely, when the reproduction light wavelength and the numerical aperture of the objective lens are determined, a diameter of beam west is decided, so that a value of about .lambda./2NA as the shortest mark length is the limit which can be reproduced. On the other hand, the track density is mainly limited by crosstalks between the adjacent tracks and depends on a spot diameter of reproducing beam in a manner similar to the shortest mark length. Therefore, to realize a high density in a conventional optical disk, it is necessary to reduce the laser wavelength of the reproducing optical system or increase the numerical aperture NA of the objective lens. However, it is difficult to reduce the wavelength of laser because of problems of efficiency, heat generation, and the like of devices. When the numerical aperture of the objective lens is increased, not only is it difficult to work the lens but also a distance between the lens and the disk becomes too short that a mechanical problem such that the lens collides with the disk or the like occurs. To prevent such a problem, a technique for improving the recording density by devising a structure of the recording medium or a reading method has been developed.
A magnetooptic reproducing method disclosed in Japanese Patent Application Laid-Open No. 3-93056 will now be described with reference to FIG. 1. FIG. 1 is a diagram for explaining an example of a conventional super-high resolution technique. The top diagram is a schematic cross sectional view showing a magnetization state of each layer of an optical disk. The middle diagram is a schematic plan view showing a mask region and an aperture region in a light spot on a plate surface of a medium. The bottom diagram is a graph showing a temperature distribution in a track direction of a corresponding portion. As shown in the diagrams, a substrate 20 is ordinarily made of a transparent material such as glass or polycarbonate. An interference layer 34, a reproduction layer 31, an intermediate layer 32, a memory layer 33, and a protective layer 35 are sequentially laminated on the substrate 20 in accordance with this order. The interference layer 34 is used to raise a Kerr effect. The protective layer 35 is used to protect a magnetic layer. An arrow in the magnetic layer indicates a direction of a magnetization or an atom magnetic moment in the film. A light spot 2 is irradiated to the medium with such a construction. A magnetic coupling of the reproduction layer 31 and memory layer 33 of a high temperature portion in a temperature distribution of the medium which occurs in this instance is cut off by the intermediate layer 32 of a low Curie temperature. The magnetization of the reproduction layer 31 of the portion in which the magnetic coupling has been cut off is aligned in one direction by an external magnetic field (reproduction magnetic field 22 in the diagram), thereby forming a rear mask 5 for partially masking the magnetic domain information of the memory layer 33 in the light spot 2. As mentioned above, an aperture 3 and the rear mask 5 corresponding to a recording mark 1 as shown in the diagram are formed in the light spot 2 on a land 7 between grooves 6a and 6b as mentioned above, thereby enabling a signal (recording mark 1) of a period that is equal to or less than a diffraction limit of the light to be reproduced and trying to improve a linear recording density.
According to super-high resolution reproducing methods disclosed in Japanese Patent Application Laid-Open Nos. 3-93058 and 4-255946, as shown in FIG. 2, by using a medium having the reproduction layer 31, intermediate layer 32, and memory layer 33, the direction of the magnetization of the reproduction layer 31 is aligned to one direction by an initialization magnetic field 21 prior to reproducing information, and the magnetic domain information of the memory layer 33 is masked. After that, the light spot 2 is irradiated. In a low temperature region in a temperature distribution of the medium which is caused in this instance, the reproduction layer 31 is allowed to maintain an initialization state (a front mask 4 is formed). In a high temperature region that is equal to or higher than a Curie temperature Tc2 of the intermediate layer 32, the reproduction layer 31 is forcedly oriented (the rear mask 5 is formed) in the direction of the reproduction magnetic field 22. In only a middle temperature region, the magnetic domain information of the memory layer 33 is transferred, thereby reducing an effective size of the reproducing spot. Thus, the recording mark 1 of the diffraction limit of the light or less is enabled to be reproduced, thereby improving the linear density.
According to a magneto-optical recording medium disclosed in Japanese Patent Application Laid-Open No. 6-124500, as a super-high resolution technique to realize a recording density that is equal to or higher than an optical resolution of the reproducing light, a medium having an interference layer 43, a reproduction layer 41, a memory layer 42, and a protective layer 44 as shown in FIG. 3 has been proposed. An arrow in a magnetic film in the diagram indicates a direction of a sub-lattice magnetization of an iron group element in the film. By "iron group" is here meant the elements iron, cobalt, and nickel. The memory layer 42 is a film such as TbFeCo, DyFeCo, or the like having a large perpendicular magnetic anisotropy. Recording information forms a magnetic domain depending on whether the magnetic domain of such a layer is upward or downward for the film surface and holds such a magnetic domain. Although the reproduction layer 41 is an in-plane magnetization film at room temperature, when a temperature rises, the reproduction layer 41 becomes a perpendicular magnetization film. When a light for reproducing information is irradiated to the medium with such a structure from the substrate 20 side, a temperature gradient at the center of a data track becomes as shown in FIG. 3. In accordance with such a temperature gradient, a low temperature region in which the reproduction layer 41 is held as an in-plane magnetization film and a high temperature region in which it becomes a perpendicular magnetization film are formed. An isothermal line of a temperature (Tl-mask) at a boundary of both of the low and high temperature regions exists as shown in FIG. 3. In the low temperature region of Tl-mask or lower, since the reproduction layer 41 becomes the in-plane magnetization film, it doesn't contribute to an extra-Kerr effect (the front mask 4 is formed) and a recording magnetic domain held in the memory layer 42 is masked and cannot be seen. On the other hand, in a temperature region of Tl-mask or higher, the reproduction layer 41 becomes the perpendicular magnetization film and the direction of the magnetization is set to the same direction as that of the recording information by the exchange coupling from the memory layer 42. Thus, since the recording magnetic domain of the memory layer 42 is transferred to only the portion of the aperture 3 smaller than the size of light spot 2, a super-high resolution is realized.
According to those well-known super-high resolution methods, since the front mask 4 in the low temperature region extends in the direction of the adjacent track, not only the linear recording density but also the track density are improved.
According to the method (FIG. 1) disclosed in Japanese Patent Application Laid-Open No. 3-93056, although the resolution can be raised without deteriorating a signal quality, it is necessary to apply a reproduction magnetic field 22. Further, according to the methods (FIG. 2) disclosed in Japanese Patent Application Laid-Open Nos. 3-93058 and 4-255946, prior to reproducing information, a magnet for initializing magnetic field 21 to align the magnetization of the reproduction layer 31 to one direction also needs to be added to the apparatus. According to the super-high resolution reproducing method (FIG. 3) disclosed in Japanese Patent Application Laid-Open No. 6-124500, since only the front mask 4 is used, there is a problem such that when the region of the mask is widened to raise the resolution, the position of the aperture 3 is deviated from the spot center, so that a signal quality deteriorates or the like.
As mentioned above, the conventional super-high resolution reproducing methods have problems such that the resolution cannot be sufficiently raised, the magnetooptic recording and reproducing apparatus becomes complicated, the costs are high, it is difficult to miniaturize, and the like.